The present invention relates to improvements of the negative electrode for constituting non-aqueous electrolyte secondary battery, more particularly to a non-aqueous electrolyte secondary battery of high energy density free from adverse internal shortcircuit as a growth factor of lithium dendrites.
There have been various vigorous studies about the non-aqueous electrolyte secondary battery including a negative electrode of lithium or lithium compound because it affords a high discharge voltage and therefore is expected to offer a high energy density. The known conventionally proposed positive electrode active materials for use in non-aqueous electrolyte secondary battery include oxides or chalcogenides of transition metal such as LiMn.sub.2 O.sub.4, LiCoO.sub.2, LiNiO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.5, MnO.sub.2, TiS.sub.2, MoS.sub.2 and the like.
The above-exemplified compounds have a layered or tunneled crystal structure permitting intercalation or deintercalation of lithium ions.
Concerning the negative electrode active material for use in non-aqueous electrolyte secondary battery, on the other hand, the use of metallic lithium has been investigated extensively. However, metallic lithium as the negative electrode active material of a battery has an inevitable issue of deposition of lithium dendrites on the surface of metallic lithium upon charge, which impairs charge and discharge efficiency or induces internal shortcircuit due to contact of the lithium dendrite with the positive electrode.
As the measure for solving such problem, there have been studies on the availability to the negative electrode of a certain lithium alloy such as lithium-aluminum alloy which is capable of absorbing therein and desorbing therefrom lithium while best reducing growth of lithium dendrites on the surface of the alloy. However, the use of such lithium alloy as the negative electrode active material has a disadvantage that the electrode material is liable to be pulverized by repetitive deep charge and discharge operations, which in turn reduces the discharge capacity of the resultant battery with the progress of charge and discharge cycles.
To solve this problem, there are proposed methods for suppressing pulverization of the electrode by using, as the electrode, a lithium alloy like lithium-aluminum alloy as mentioned above further containing other elements. These proposals can be found in Japanese Laid-Open Patent Publications No. Sho 62-119856 and Hei 4-109562. However, these attempts have failed to achieve a lithium ion secondary battery including a lithium alloy negative electrode fulfilling the required characteristics or performance in practical use.
Instead, the presently realized lithium ion secondary battery includes a carbon material as the negative electrode active material in place of the above-mentioned alloy system.
Carbon material affords a smaller capacity than the alloy negative electrode active material but can reversibly absorb therein and desorb therefrom lithium, thus manifesting an excellent charge and discharge cycle life. Another advantage of carbon material is unlikeliness to develop deposition of lithium dendrites on the surface of the negative electrode active material upon charge, thereby facilitating security of the safety of the resultant battery with relative ease.
Under the circumstance, there is a proposal of a lithium ion secondary battery including an oxide negative electrode in order to realize a battery having a further increased capacity. The proposed oxides as the negative electrodes affording a higher capacity than the conventional compound such as WO.sub.2 may be exemplified as SnO and SnO.sub.2 of crystalline structure, which are disclosed in Japanese Laid-Open Patent Publications No. Hei 7-122274 and Hei 7-235293. The use of an amorphous oxide such as SnSiO.sub.3 or SnSi.sub.1-x P.sub.x O.sub.3 for the negative electrode has also been proposed in Japanese Laid-Open Patent Publication No. Hei 7-288123 in order to improve the cycle life characteristics of a battery.
The present inventors suggest that metal salts or semi-metal salts comprising at least one selected from the group consisting of nitrate, sulfate, hydrogen sulfate, thiocyanate, cyanide, cyanate, carbonate, hydrogen carbonate, hydrogen borate, hydrogen phosphate, selenate, hydrogen selenate, tellurate, hydrogen tellurate, tungstate, molybdate, titanate, chromate, zirconate, niobate, tantalate, manganate, and vanadate can serve as the negative electrode active materials producing a high capacity non-aqueous electrolyte secondary battery with an excellent cycle life characteristic.
The present inventors also suggest that compounds of crystalline structure containing at least two or more selected from the group consisting of Si, Ge, Sn, Pb, Bi, P, B, Ga, In, Al, As, Sb, Zn, Ir, Mg, Ca, Sr, and Ba together with at least one selected from the group consisting of oxygen, sulfur, selenium, and tellurium can constitute negative electrode materials giving a non-aqueous electrolyte secondary battery having a high capacity and an excellent cycle life characteristic.
The present inventors experimentally confirmed a drastic improvement of the cycle life characteristic in the non-aqueous electrolyte secondary batteries including the above-exemplified negative electrode active materials compared to that of the non-aqueous electrolyte secondary batteries having conventionally proposed materials.
However, in correspondence with the current active movement toward realization of a multi-functional portable equipment and a large battery for use in an electric vehicle, for example, there is an increasing demand for a battery of an even longer life as a driving power source. It is true, however, that the above-mentioned negative electrode materials have not yet fulfilled the demand for a battery having a sufficiently long cycle life.
In view of such current demand, the object of the present invention is, therefore, to provide a high capacity non-aqueous electrolyte secondary battery which is free of growth of adverse lithium dendrites during charge and discharge and has a drastically elongated cycle life.